node: refactor package node

[renaynay]

This PR significantly changes the APIs for instantiating Ethereum nodes in a Go program. The new APIs are not backwards-compatible, but we feel that this is made up for by the much simpler way of registering services on node.Node. You can find more information and rationale in the design document: https://gist.github.com/renaynay/5bec2de19fde66f4d04c535fd24f0775.

There is also a new feature in Node's Go API: it is now possible to register arbitrary handlers on the user-facing HTTP server. In geth, this facility is used to enable GraphQL.

There is a single minor change relevant for geth users in this PR: The GraphQL API is no longer available separately from the JSON-RPC HTTP server. If you want GraphQL, you need to enable it using the --http --graphql flag combination. The --graphql.port and --graphql.addr flags are no longer available.

---

[ + ] Introduces Lifecycle interface
[ + ] Introduces RegisterAPIs() method
[ + ] Introduces RegisterLifecycle() method
[ + ] Introduces RegisterHTTP() method
[ + ] Introduces RegisterProtocols() method

[<>] Changes the way information about node's HTTP servers is stored
[<>] Separates LES server management from eth.Ethereum, now managed by node
[<>] GraphQL can only be started on the node's canonical HTTP server (to enable graphql --graphql, the user must also enable http with the --http flag)

[ - ] Removes Service interface
[ - ] Removes ServiceContext
[ - ] Removes the responsibility of service construction

TODO

• functional implementation of design
• fix RegisterHTTP methods
• general code clean-up
  • clean up code in cmd/geth/main.go
  • clean up services to which 2 backends are passed (where one is nil)
• tests
• make sure documentation is accurate
• rebase
• clean-up

[renaynay]

Notes from review:

• either add a method to get consensus.Engine from the ethapi.Backend or get it another way.
• remove all "fetch backend" instances and try to remove the Lifecycle method where it fetches a lifecycle based on its type.
• remove ServiceContext
• change the panic of missing backend to a nicer fatal err
• take another look at WSAllowed because there could be a resulting race condition
• RegisterHTTP could possibly be done better and could maybe still follow this signature:

func (n *Node) RegisterHTTP(path string, h http.Handler) error

• Also fix GQL server tests, they're still failing on some builds on travis.

[fjl]

I think everything in this block is code that only works for full nodes.
We should create an auxiliary function for setting up *eth.Ethereum specifically
and then put a type assertion for *eth.Ethereum here.

[renaynay]

HTTP Rule:

flags:
--http
--http.port
--http.cors
--http.vhosts
--http.graphql enables graphql on http server

• can graphql be enabled without http enabled? NO
• graphql can only be on http server, not on separate endpoint.

console:

• if I do a stopRPC() in the console, what happens to enabled gql server? It also stops as well.

[renaynay]

TODO:

• ethstats gets own backend interface + full node backend then do type assertion (don't import internal ethapi)
• atomic int32 wsAllowed
• real type assertion for main.go so that configure eth vs les stuff in startNode
• graphql can only be on http

extra:

• SetContractBackend move this to eth / les configuration (ipc endpoint)